Aluminum reflecting mirror and method of making the same

ABSTRACT

Within a chamber in a vacuum atmosphere, SiO is deposited on a glass substrate so as to form a first layer as a protective film, Al is deposited on the first layer so as to form a second layer as an aluminum reflective film, MgF 2  is deposited on the second layer so as to form a third layer as a transparent protective film, and CeO 2  is deposited on the third layer so as to form a fourth layer as a transparent protective film. Then, while O 2  gas is introduced into the chamber, SiO 2  is deposited on the fourth layer so as to form a fifth layer as a transparent protective film.

RELATED APPLICATIONS

This application claims the priority of Japanese Patent Application No.2000-167485 filed on Jun. 5, 2000, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of making an aluminumreflecting mirror in which a protective film, an aluminum reflectivefilm, and a transparent protective film comprising a plurality of layersare laminated on a glass substrate; and to the aluminum reflectingmirror made in accordance with the method to which the inventionrelates.

2. Description of the Prior Art

Various kinds of aluminum reflecting mirrors for use in opticalapparatus have conventionally been developed.

For example, Japanese Unexamined Patent Publication No. 5-150105discloses an aluminum reflecting mirror in which a surface of asubstrate made of glass, plastics, or ceramics is smoothed, a protectivefilm made of SiO₂ is formed on the surface of substrate, a reflectivefilm made of Al is formed on the protective film, and a transparentprotective film is formed on the reflective film, whereas thetransparent protective film comprises an MgF₂ film, a TiO₂ or CeO₂ film,and an Al₂O₃ film.

When Al is directly deposited on a glass substrate, the Al reflectivefilm may be corroded by ingredients precipitated from the glasssubstrate. Therefore, the SiO₂ film is formed between the glasssubstrate and aluminum reflective film in the aluminum reflecting mirrordisclosed in the above-mentioned publication, so as to prevent thealuminum reflective film from corroding, for example.

Also, the transparent protective film in which an MgF₂ film, a TiO₂ orCeO₂ film, and an Al₂O₃ film are laminated on the aluminum reflectivefilm is formed in order to protect the aluminum reflecting film andimprove its environmental resistance.

While Al₂O₃ is deposited as the outermost transparent protective film inthe above-mentioned conventional aluminum reflecting mirror, Al₂O₃cannot be deposited by resistance-heating deposition method but byelectron bombardment method using an expensive electron gun, whichincreases the cost of equipment.

Also, while the deposition of each layer and the like must be carriedout in a vacuum atmosphere when making an aluminum reflecting mirror, ittakes a certain extent of time for attaining the vacuum atmospherewithin a chamber. Therefore, for improving the manufacturing efficiency,it is necessary that all the materials for forming the substrate,protective film, aluminum reflective film, and transparent protectivefilm be accommodated within the chamber and then the chamber beevacuated, so as to continuously carry out the deposition of individuallayers and the like.

Since different materials are used for the protective film, aluminumreflective film, and individual layers of the transparent protectivefilm, however, a space for accommodating all these materials isnecessary in the above-mentioned conventional aluminum reflectingmirror. Further, instruments used for resistance-heating depositionmethod and an electron gun must be provided, which increases the size ofchamber.

SUMMARY OF THE INVENTION

In view of the circumstances mentioned above, it is an object of thepresent invention to provide a method of making an aluminum reflectingmirror by which all the layers can be formed by resistance-heatingdeposition method whereas the size of manufacturing equipment can bereduced, and an aluminum reflecting mirror.

In order to achieve the above-mentioned object, the present inventionprovides a method of making an aluminum reflecting mirror in which firstto fifth layers are laminated on a glass substrate successively from theglass substrate side, the method comprising:

a step of evacuating a chamber accommodating therein the glass substrateand respective materials for forming the first to fifth layers;

a step of forming the first layer as a protective film by depositing SiOon the glass substrate;

a step of forming the second layer as an aluminum reflective layer bydepositing Al on the first layer;

a step of forming the third layer as a transparent protective layer bydepositing MgF₂ on the second layer;

a step of forming the fourth layer as a transparent protective layer bydepositing CeO₂ on the third layer; and

a step of forming the fifth layer as a transparent protective layer bydepositing SiO₂ on the fourth layer while introducing O₂ gas into thechamber.

The step of forming the third layer may comprise:

a first step of depositing MgF2 at a deposition temperature lower than80° C.; and

a second step of depositing MgF2 at a deposition temperature higher than300° C.

Each of the first to fifth layers may be formed by resistance-heatingmethod.

The step of forming of the third layer may be carried out such that alower layer thereof is formed by lower-temperature deposition, and thenan upper layer thereof is formed by higher-temperature deposition.

The present invention provides an aluminum reflecting mirror in whichfirst to fifth layers are laminated on a glass substrate successivelyfrom the glass substrate side;

wherein the first layer is a protective film formed by depositing SiO,the second layer is an aluminum reflective film formed by depositing Al,the third layer is a transparent protective film formed by depositingMgF₂, the fourth layer is a transparent protective film formed bydepositing CeO₂, and the fifth layer is a transparent protective filmformed by depositing SiO₂.

The third layer may comprise a two-layer structure composed by a lowerlayer formed by lower-temperature layer and an upper layer formed by ahigher-temperature deposition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the layer structure of the aluminumreflecting mirror in accordance with an embodiment of the presentinvention;

FIG. 2 is a flowchart showing the procedure of making the aluminumreflecting mirror in accordance with the embodiment;

FIG. 3 is a graph showing reflection characteristics in a visible lightregion of the aluminum reflecting mirror in accordance with theembodiment;

FIG. 4 is a graph showing reflection characteristics in the visiblelight region of an aluminum reflecting mirror acting as a comparativeexample for an environmental resistance test; and

FIG. 5 is a schematic view showing the layer structure of the aluminumreflecting mirror acting as the comparative example for theenvironmental resistance test.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, the aluminum reflecting mirror in accordance with anembodiment of the present invention will be explained with reference tothe drawings.

FIG. 1 is a schematic view showing the layer structure of the aluminumreflecting mirror in accordance with the embodiment of the presentinvention.

As shown in FIG. 1, the aluminum reflecting mirror in accordance withthe embodiment of the present invention comprises a glass substrate 1;and a first layer as a protective film 2 formed by depositing SiO, asecond layer as an aluminum reflective film 3 formed by depositing Al, athird layer as a transparent protective film formed by depositing MgF₂,a fourth layer as a transparent protective film 5 formed by depositingCeO₂, and a fifth layer as a transparent protective layer 6 formed bydepositing SiO₂ while introducing O₂ gas into a chamber, which aresuccessively laminated on the glass substrate 1 in this order therefrom.

The individual layers have such thickness values that, when thewavelength of light in use λ=633 nm, the first layer as the protectivefilm 2 has an optical film thickness of (¼)λ to (½)λ, the second layeras the aluminum reflective film 3 has a mechanical film thickness of 100to 500 nm, the third and fourth layers as the transparent protectivefilms 5, 6 each have an optical film thickness of (¼)λ, and the fifthlayer as the transparent protective film 6 has an optical film thicknessof (⅛)λ to ({fraction (1/16)})λ.

Now, with reference to FIG. 2, a method of making the above-mentionedaluminum reflecting mirror will be explained.

FIG. 2 is a flowchart showing the procedure of making the aluminumreflecting mirror in accordance with this embodiment.

First, when making the aluminum reflecting mirror in accordance with theembodiment of the present invention, the glass substrate 1 isaccommodated in a chamber, and materials for the first layer as theprotective film 2, the second layer as the aluminum reflective film 3,and the third to fifth layers as the transparent protective layers 4, 5,6 are also accommodated therein.

Subsequently, as shown in FIG. 2, the chamber is evacuated (S1), suchthat the pressure therein becomes 3×10⁻⁵ hPa or less (S2). In thisstate, the first layer as the protective film 2, the second layer as thealuminum reflective film 3, and the third to fifth layers as thetransparent protective films 4, 5, 6 are formed on the glass substrate 1successively from the glass substrate 1 side by using resistance-heatingdeposition method.

First, SiO is deposited with an optical film thickness of (¼)λ to (½)λ,so as to form the first layer as the protective film 2 (S3).Subsequently, Al is deposited with a mechanical thickness of 100 to 500nm on the first layer as the protective film 2, so as to form the secondlayer as the aluminum reflective film 3 (S4).

Then, MgF₂ to become the third layer as the transparent protective film4 is deposited. Here, MgF₂ is initially deposited on the second layer asthe aluminum reflective film 2 at a deposition temperature lower than80° C. (S5), and then at a deposition temperature higher than 300° C.(S6), so that the total optical film thickness becomes (¼)λ. These steps(S5, S6) yield substantially the same film thickness.

The transparent protective film of the third layer is thus formed bytwo-stage deposition in order for the lower-temperature in step 5 (S5)to prevent the second layer as the aluminum reflective film 3 fromlowering its reflectivity, and for the higher-temperature deposition instep 6 (S6) to strengthen the film. Further, CeO₂ is deposited with anoptical film thickness of (¼)λ, so as to form the fourth layer as thetransparent protective film 5 (S7).

After the fourth layer as the transparent protective film 5 iscompletely formed, the pressure within the chamber is set to 3×10⁻⁴ hPa,O₂ gas is introduced, and SiO₂ is deposited with an optical filmthickness of (⅛)λ to ({fraction (1/16)})λ, so as to form the fifth layeras the transparent protective film 6 (S8).

The environmental resistance of the aluminum reflecting mirror inaccordance with this embodiment will now be explained.

An environmental resistance test was carried out for the aluminumreflecting mirror in accordance with this embodiment and a conventionalaluminum reflecting mirror.

FIG. 5 is a vertical sectional view of the aluminum reflecting mirroracting as a comparative example for the environmental resistance test.

As shown in FIG. 5, the aluminum reflecting mirror acting as thecomparative example comprises a glass substrate 11; and a first layer asan aluminum reflective film 12 formed by depositing Al, a second layeras a transparent protective film 13 formed by depositing MgF₂, and athird layer as a transparent protective film 14 formed by depositingCeO₂, which are successively laminated on the glass substrate 11 in thisorder therefrom.

The individual layers have such thickness values that, when thewavelength of light in use λ=633 nm, the first layer as the aluminumreflective film 12 has a mechanical film thickness of 100 to 500 nm, andthe second and third layers as the transparent protective films 13, 14each have an optical film thickness of (¼)λ.

Here, as mentioned above, the film thickness of the aluminum reflectingmirror in accordance with this embodiment used in the environmentalresistance test is such that the first layer as the protective film 2has an optical film thickness of (¼)λ to (½)λ, the second layer as thealuminum reflective film 3 has a mechanical film thickness of 100 to 500nm, the third and fourth layers as the transparent protective films 5, 6each have an optical film thickness of (¼)λ, and the fifth layer as thetransparent protective film 6 has an optical film thickness of (⅛)λ to({fraction (1/16)})λ.

Test items in the environmental resistance test are “Adhesion”(MIL-C-675C) which is a test for resistance to peel, “Moderate abrasion”(MIL-C-675C) which is a test for resistance to wear, “Humidity”(MIL-C-675C) which is a test for resistance to moisture, and “Salt sprayfog” (MIL-C-675C) which is a test for resistance to salt.

The contents of individual test items are as follows:

In the test for resistance to peel, a defined width of cellophane tapewas firmly attached to the surface of reflecting mirror, and thenrapidly peeled off, whereby the resistance to peel was studied.

In the test for resistance to wear, the surface of reflecting mirror wasrubbed 50 strokes with a defined size of cheesecloth with a load of 1pound applied thereto, whereby the resistance to wear was studied.

In the test for resistance to moisture, the reflecting mirror was leftfor 24 hours in an environment of 50° C. and 95% Rh, whereby theresistance to humidity was studied.

In the test for resistance to salt, the reflecting mirror was left for24 hours in an environment in which 5% NaCl was sprayed into anatmosphere of 35° C., whereby the resistance to salt was studied.

The results of tests are shown in the following Table 1.

As can be seen from the above-mentioned Table 1, both the aluminumreflecting mirror in accordance with this embodiment and that of thecomparative example exhibited favorable results in the tests forresistance to peel, resistance to wear, and resistance to salt.

In the test for resistance to moisture, however, the mirror surfacediscolored and deteriorated in the aluminum reflecting mirror of thecomparative example. By contrast, the aluminum reflecting mirror inaccordance with this embodiment exhibited neither discoloration nordeterioration even after the lapse of 96 hours or more.

As mentioned above, the protective film 2 for blocking precipitates fromthe glass substrate 1 in order to protect the aluminum reflective film 3is formed from SiO in the aluminum reflecting mirror in accordance withthis embodiment. This SiO film is denser than the SiO₂ film, therebybeing able to more securely block precipitates from the glass substrate1 and prevent the metal reflective film from corroding.

Also, the third layer as the transparent protective film 4 is formedfrom the MgF₂ film, whereas the fourth layer as the transparentprotective film 5 is formed from the CeO₂ film. Since MgF₂ and CeO₂ haverespective stresses acting in opposite directions, the adhesion betweenthese transparent protective films can be enhanced.

FIG. 3 shows reflection characteristics (P-polarized light, S-polarizedlight, and their average value) in a visible light region of thealuminum reflecting mirror in accordance with this embodiment, whereasFIG. 4 shows those of the aluminum reflecting mirror in accordance withthe comparative example. In each of FIGS. 3 and 4, the abscissaindicates the wavelength (nm) of incident light, whereas the ordinateindicates the light reflectivity (%).

While the aluminum reflecting mirror in accordance with this embodimenthas the transparent protective film 6 made of SiO₂ in the outermostfifth layer as compared with the aluminum reflecting mirror inaccordance with the comparative example, the reflection characteristicsof the former are as favorable as those of the latter as can be seenfrom FIGS. 3 and 4.

In the aluminum reflecting mirror in accordance with the presentinvention, the film thickness of each layer can be changed appropriatelywithout being restricted to those mentioned above.

In the method of making an aluminum reflecting mirror and the aluminumreflecting mirror in accordance with the present invention, as explainedin the foregoing, an SiO film is formed as the first layer acting as aprotective film, an aluminum reflective film is formed as the secondlayer, an MgF₂ film is formed as the third layer acting as a transparentprotective film, a CeO₂ film is formed as the fourth layer acting as atransparent protective film, and an SiO₂ film is formed as the fifthlayer acting as a transparent protective film. Consequently, thedeposition can be carried out in all the layers by resistance-heatingdeposition method without using expensive electron guns, whereby thecost of equipment can be reduced.

Also, since the first layer as a protective film and the fifth layer asa transparent protective film are formed from SiO and SiO₂,respectively, which comprise Si as a material, the kinds of materialsaccommodated in the chamber can be reduced, whereby the chamber can bemade smaller.

Since the chamber is evacuated in the initial step, whereas O₂ isintroduced into the chamber in the final step, the individualfilm-forming steps can be carried out efficiently and continuously,whereby the working efficiency can be enhanced.

TABLE 1 Test item Description of test Comp. Ex. Embodiment Adhesion Adefined width of cellophane good good MIL-C-675C tape is firmly attachedand then peeled rapidly. Moderate Rubbed 50 strokes with a load goodgood abrasion of 1 pound applied thereto. MIL-C-675C Humidity Left for24 hr at 50° C., 95% neither good MIL-C-675C Rh. good nor bad* Saltspray fog Left for 24 hr in spray of 5% good good MIL-C-675C NaCl at 35°C. *The mirror surface discolors and deteriorates after 24 hours in thecomparative example, whereas no change is seen in the embodiment evenafter 96 hours.

What is claimed is:
 1. A method of making an aluminum reflecting mirrorin which first to fifth layers are laminated on a glass substratesuccessively from said glass substrate side, said method comprising: astep of evacuating a chamber accommodating therein said glass substrateand respective materials for forming said first to fifth layers; a stepof forming said first layer as a protective film by depositing SiO onsaid glass substrate; a step of forming said second layer as an aluminumreflective layer by depositing Al on said first layer; a step of formingsaid third layer as a transparent protective layer by depositing MgF₂ onsaid second layer in two substeps, a first substep of depositing MgF₂ ata deposition temperature lower than 80° C.; and a second substep ofdepositing MgF₂ at a deposition temperature higher than 300° C.; a stepof forming said fourth layer as a transparent protective layer bydepositing CeO₂ on said third layer; a step of forming said fifth layeras a transparent protective layer by depositing SiO₂ on said fourthlayer while introducing O₂ gas into said chamber.
 2. A method of makingan aluminum reflecting mirror according to claim 1, wherein said firstto fifth layers are formed by resistance-heating method.
 3. A method ofmaking an aluminum reflecting mirror according to claim 1, wherein saidevacuating step evacuates said chamber to a high level of vacuum andsaid first through said fourth layer forming steps are carried out atsaid high level of vacuum and said fifth layer forming step is carriedout at a lower level of vacuum.
 4. A method of making an aluminumreflecting mirror in which first to fifth layers are laminated on aglass substrate successively from said glass substrate side, said methodcomprising: a step of evacuating a chamber accommodating therein saidglass substrate and respective materials for forming said first to fifthlayers; a step of forming said first layer as a protective film bydepositing SiO on said glass substrate; a step of forming said secondlayer as an aluminum reflective layer by depositing Al on said firstlayer; a step of forming said third layer as a transparent protectivelayer by depositing MgF₂ on said second layer in two substeps, a lowerlayer thereof is formed in a first substep by lower-temperaturedeposition, and then an upper layer thereof is formed in a secondsubstep by higher-temperature deposition, and wherein saidlower-temperature deposition is carried out at a temperature to preventsaid second aluminum reflective layer from lowering its reflectivity andsaid higher-temperature deposition is carried out at a temperature tostrengthen said third layer; a step of forming said fourth layer as atransparent protective layer by depositing CeO₂ on said third layer; astep of forming said fifth layer as a transparent protective layer bydepositing SiO₂ on said fourth layer while introducing O₂ gas into saidchamber.
 5. A method of making an aluminum reflecting mirror accordingto claim 4, wherein said evacuating step evacuates said chamber to ahigh level of vacuum and said first through said fourth layer formingsteps are carried out at said high level of vacuum and said fifth layerforming step is carried out at a lower level of vacuum.